The tropical cyclone genesis potential index over the western North Pacific and its interannnal variability as simulated by the LASG/IAY ALCM

The tropical cyclone genesis potential index (UPI) is a useful metric for gauging the performance of global climate models in the simulation of tropical cyclone genesis. The authors have assessed the performance of the LASU/IAP AUCM UAMIL2.0 in the simulation of UPI over the western North Pacific (WNP). Since the value of UPI is determined by large scale environmental factors including the low-level vorticity (850 hPa),the relative humidity at 700 hPa, the magnitude of vertical wind shear between 850 and 200 hPa, the maximum the potential intensity (MPI) and the vertical velocity, the bias of the model in the UPI simulation is discussed by analyzing these foregoing thermal and dynamical factors. The model results arc compared to the ERA-40 rcanalysis data. The results show that both the spatial pattern and the seasonal cycle of the UPI distribution in the WNP arc reasonably simulated by the UAMIL2.0. Due to the overestimate of the relative humidity, the simulated UPI extends eastward to 170°E, which shifts eastward about 10° relative to the rcanalysis. Another deficiency of the model is that the monsoon trough is about 5° north to the rcanalysis;the model overestimates the UPI in the early season (May-June) and later season (Sep-Oct),but underestimates the UPI in the mature season (July-Aug). For the interannual variation,the response of GPI to ENSO in WNP is well simulated,including the eastward shift of genesis location during the El Niño phase, and westward shift during the La Niña phase. One limitation of the simulation is that the anomalous connective center shifts westward about 20° in comparison to the rcanalysis, which leads to the biases of both vertical velocity and relative humidity distribution. Thus the boundary between positive and negative bias zones over 20°-30°N shifts westward. The analysis has provided a useful reference for the future improvement of the model.